A single instrument aboard SNAP, the proposed
SuperNova/ Acceleration Probe, will make it unique among satellites: its
billion-pixel astronomical camera, the GigaCAM. Built from an array of revolutionary
Berkeley Lab CCDs developed by Stephen Holland and his colleagues in the
Lab's Engineering and Physics Divisions, the GigaCAM will be the largest
and most sensitive astronomical CCD imager ever constructed.

SNAP will search for supernovae from
a high polar orbit.

Standard astronomical CCDs are fragile affairs, and their ability to
obtain high-quality images degrades quickly in the hostile radiation environment
of space -- one reason why astronauts have already replaced all of the
Hubble Space Telescope's original imaging instruments.

"The Hubble and many other satellites were designed to be maintenance
friendly, but SNAP is going to be placed in an unreachable orbit,"
says Engineering's William Kolbe, noting that once the GigaCAM is carried
aloft on its five-year-plus mission, it can't afford to fail.

At 300 microns (millionths of a meter) thick, the Berkeley Lab "high-resistivity,
p-channel" CCDs are much more rugged than conventional astronomical
CCDs measuring only a few tens of microns thick. In recent months Kolbe
and Armin Karcher have been conducting tests at the 88-Inch Cyclotron
to see just how well the new CCD can stand up to radiation damage.

Outer space in the lab
In space the culprits are cosmic rays, high-velocity particles packing
a tremendous energetic punch that destroys pixels, increases "dark
current" (a source of background noise), and worst of all degrades
the efficiency of charge transfer from the pixels to the amplifiers at
the edges of the chip. A few cosmic rays are massive atomic nuclei like
iron, nickel, or zinc, but the majority are protons and electrons.

The 88-Inch Cyclotron simulates the cosmic ray environment with both heavy
ions and protons, to test spacecraft components ranging from memory chips
to transistors to entire computer systems, and to calibrate detectors
used in compiling space "weather" reports. CCD chips and solar
cells are particularly prone to degradation from large numbers of protons
generated during high solar activity.

The Light Ion Irradiation Station
located in the 88-Inch Cyclotron's Cave 3, originally developed for
radiation biology, is often used to test spacecraft components

"At the Cyclotron, protons are delivered to target components at
the Light Ion Irradiation Station located in Cave 3," says Peggy
McMahan, research coordinator for the 88-Inch Cyclotron. "We owe
this station to a group from the Life Sciences and Engineering Divisions,
who worked with our operations staff to develop it in order to maintain
a small radiation biology program here after the closure of the Bevalac
in 1993."

In the station the dose is measured and the beam is "blown up"
to four inches in diameter to uniformly irradiate silicon wafers (and
the Petrie dishes used in Life Sciences experiments too). The Materials
Sciences, Chemical Sciences, and Advanced Light Source divisions have
also used the irradiation facility when the cyclotron is not busy with
nuclear science experiments, its primary mission. Companies who have tested
components for space applications on a fee basis include Eastman Kodak,
Aerospace Corporation, Lucent Technologies, and Mitsubishi Electronics.

Test CCDs for the SNAP project are bombarded with beams of protons ranging
in energy from 10 to 55 MeV (million electron volts); by testing several
wafers, each at a different dose -- from a few billion to a hundred billion
protons per square centimeter of surface -- dosage can be scaled to equal
what CCDs would receive after several years in orbit.

Performance check
Before irradiation, Kolbe and Karcher assess the test wafers in their
laboratory for dark current, charge transfer efficiency, and "cosmetic"
defects. Computer processing and other electronic tricks can compensate
for cosmetic flaws like a few damaged ("hot") pixels, endemic
to all CCDs, but dark current and charge transfer efficiency pose more
serious challenges.

Dark current is electronic noise caused by thermal motion of the atoms
that make up the chip; the colder the chip, the less the dark current.
The Berkeley Lab CCD is much thicker than ordinary astronomical chips
so there is more material in which dark current can be generated, but
its high purity, negative "doping," and low operating temperature
work to suppress dark current. In space, SNAP's GigaCAM will operate at
about 140 degrees Kelvin (by comparison, nitrogen under normal atmospheric
pressure liquefies at 77 deg K).

The operating temperature of SNAP's
GigaCAM will be about 140 degrees Kelvin, a low temperature that will
help reduce dark current.

Typically the most serious radiation damage to CCDs is a steady reduction
in charge transfer efficiency. Photons from distant objects like stars
are focused on pixels in the CCD, and the brighter the object, the more
photons are converted to charge. Negative electrons or positive "holes"
are collected and transferred to the edge of the chip along specific channels,
like buckets of water in a bucket brigade. The chip's electronics reconstruct
the image of a star by associating the precise amount of charge and the
precise location of the pixels that generated it.

"When you irradiate a CCD with protons, silicon nuclei are knocked
out of their lattice position, and what was fairly perfect material develops
defects," Kolbe says. "These form electron or hole 'traps' that
can grab charge that's being transferred, hold onto it for a time, then
let it go later."

Armin Karcher notes that these inefficiencies in charge transfer "can
affect the apparent brightness of objects in the sky and the interpretation
of their spectra." The success of the SNAP satellite will greatly
depend on its ability to measure supernova spectra with extreme accuracy.

To characterize the test CCDs, Kolbe and Karcher create star fields by
exposing them to a small x-ray source. "Each x-ray photon deposits
an artificial 'star' every 50 to 70 pixels, generating a cloud of charge,"
says Karcher. "We know how many electrons it takes to represent each
of these, so when we read out the data we know what the reading should
be."

And the winner is . . .
After irradiation at the 88-Inch Cyclotron, Kolbe and Karcher take the
test wafers back into the laboratory to measure radiation effects from
different doses. In the three batches tested so far they found that, while
radiation dosage increased dark current, the effect was important only
at high temperatures -- and the impact of radiation on charge transfer
efficiency was remarkably small. Available studies indicate that the charge
transfer efficiency of conventional CCDs falls off rapidly with increased
radiation, while the Berkeley Lab CCDs are little affected even at very
high doses.

These results were not unexpected; after all, the Berkeley Lab CCD descends
from a long line of detectors designed to withstand radiation from colliding
beams of particles in giant research accelerators -- "much more hostile
than outer space," Kolbe remarks.

"The high-resistivity p-channel CCDs exhibit extremely low dark
current at the operating temperature," the researchers concluded
in their latest report. "Radiation damage proved to be much less
detrimental than in conventional CCDs. . . . Their potential lifetime
in space is measured in decades, not years."

Kolbe and Karcher have devised new instruments to test larger CCD wafers,
to measure the efficiency of their response at all wavelengths, and to
investigate what effect pixel size, different levels of doping, and other
manufacturing variables may have on their performance after radiation
exposure.